There have been known a number of sheet transport devices for use in a printer or like image forming apparatus in which data such as addresses successively output from a data storage provided in a personal computer or the like are printed on printing material. One of those sheet transport devices is, as shown in FIGS. 10 and 11, constructed such that sheets contained in two containers 101, 102 defined in a cassette 100 are dispensed and transported side by side by a feed roller 104 including a plurality of roller members fixed to a drive shaft 103 which is drivingly rotated by unillustrated drive means.
In the sheet transport device as described above, sheets dispensed from the cassette 100 are fed by the feed rollers 104 and transported to a pair of transport rollers 108 by way of a pair of separating rollers 107. The separating roller pair 107 include a forward roller 105 and a retard roller 106 which are respectively rotatable in directions along and against transport of sheet. Thereafter, registration sensors 109 disposed in specified positions along respective sheet transport paths detect the presence of the sheets fed from the containers 101, 102. Upon lapse of a predetermined period following detection of the presence of both sheets by the sensors 109, a pair of registration rollers 110 are driven to transport the sheets to a photosensitive drum 111. In synchronism with the driving of the registration rollers 110, the driving of the feed roller 104 is stopped.
There are cases where the sheets 112a, 112A may be dispensed from the containers 101, 102 at different timings for some causes and transported to a downstream side with one sheet 112A slightly delayed relative to the other 112a, i.e. misaligned, as shown in FIG. 12. However, even if the sheets are transported toward the registration rollers 110 in this state, the registration rollers 110 are not driven until the presence of both sheets 112a, 112A is detected by the registration sensors 109 as described above. Accordingly, the transport of the sheet 112a going ahead is suspended with the leading edge of the sheet 112a at the registration rollers 110 until the presence of the delayed sheet 112A is detected by the corresponding sensor 109. The delayed sheet 112A reaches the registration rollers 110 while the transport of the sheet 112a is suspended, thereby correcting the misalignment of sheets. Normally, the misalignment caused in dispensing the sheets from the containers is corrected in the above manner. However, since the transport rollers 108 are kept rotating while the transport of the sheet 112a is suspended, a trailing edge of the sheet 112a is kept transported while warping. Accordingly, although the leading edges of the sheets 112a and 112A are aligned by operational relationship of the registration rollers 110 and registration sensors 109, the trailing edges thereof are kept misaligned. More specifically, the sheets 112a, 112A may be nipped by the feed roller 104 with the trailing edges thereof misaligned. Alternatively, as shown in FIG. 12, the trailing edge of the sheet 112a may be released from the feed roller 104 while that of the sheet 112A may be nipped thereby. If the driving of the feed roller 104 is stopped in these states, the respective sheets 112a, 112A are pulled and moved by rotation of the transport rollers 108. At this time, a roller member 104A of the feed roller 104 in contact with the delayed sheet 112A rotates due to the frictional force with the sheet 112A and the drive shaft 103 rotates together with the roller member 104A. Accordingly, a next sheet 112b contained in the container 102 is dispensed by an amount of misalignment of the sheets 112a, 112A according to rotation of the drive shaft 103 since the roller member 104a is fixed to the drive shaft 103. Thus, the misalignment of sheets occurs in the succeeding sheet transport as well. Particularly in a continuous printing operation, once the misalignment occurs, it occurs in succession. This may cause frequent occurrences of a jam, smear, and damage of sheets.
Further, the above sheet transport device in which the sheets are transported side by side suffers the following problems. In the case where one of the sheets is jammed when dispensed by the feed roller, the driving of the feed roller is stopped to facilitate removal of the jammed sheet. When the jammed sheet is pulled out of the container in a sheet transport direction, the roller member of the feed roller in contact with the jammed sheet and the drive shaft rotate together, resulting in rotation of the other roller member. Accordingly, the sheet contained in the other container is inadvertently dispensed according to rotation of the drive shaft.
The same problem arises in the case where either one of the sheets is jammed while being transported after fed from the containers. When the jammed sheet is removed, the other sheet is inadvertently pulled out together with the jammed sheet.